The present invention relates to a silver and silver alloy plating bath. The present invention provides a bath with excellent stability over an extended time. With regard to silver alloy plating baths, the present invention provides a safe, non-cyanide bath, which can reliably codeposit silver and another metal.
In general, silver readily forms an insoluble salt with various compounds. As a result, it is difficult to dissolve silver in a plating bath in a manner that is stable over an extended time. Decomposition of the bath and deposition of silver occurs readily. Furthermore, silver is an electrochemically noble metal, and as a result, alloy plating with other metals is difficult. Because of this, there are limitations on the types of silver plating baths that are practical. For example, in silver or silver-tin alloy plating baths, alkaline cyanide baths, containing various cyanide compounds, are conventionally known.
Cyanide compounds, however, are extremely poisonous. Therefore, special waste water treatment is required. This results in a rise in treatment costs. Furthermore, because these baths can only be used in the alkaline range, the types of companion metals are limited when conducting silver alloy plating. In addition, with alkaline baths, there are limitations on its uses, and in practical terms, these cyanide baths do not have adequate stability.
As a result, there is a need for development of a new silver or silver alloy plating bath, which is highly safe and in which silver can be dissolved in a stable manner over a wide pH range including strongly acidic pH's.
In Japanese Laid-Open Patent Publication Number 9-143786 (henceforth referred to as JP '786), a non-cyanide silver plating bath is disclosed. The plating bath of JP '786 is a silver plating bath, or a silver alloy plating bath, such as a silver-tin alloy, silver-copper alloy, silver-indium alloy, and the like, containing thioglycol, thioglycolic acid, thiodiglycolic acid, beta-thiodiglycol, dibenzothiazole disulfide, 4,4′-thiobis (3-methyl-6-tert-butylphenol), thiourea, and the like.
In the conventional plating bath of JP '786, by having the plating bath contain a specified sulfur-containing compound, such as thiodiglycolic acid, beta-thiodiglycol, dibenzothiazole disulfide, or thiourea, and the like, the plate coating has a fineness similar to that achieved by conventional cyanide plating baths.
However, for example, with the above silver-tin alloy plating bath containing thiodiglycolic acid or beta-thiodiglycol and the like, in reality, there is often decomposition of the bath, including deposition of silver, in 2-4 weeks. As an electric plating bath for long-term, continuous usage, there are practical problems in its stability over an extended time.
Furthermore, when current density conditions are changed, the rate of codeposition of silver can fluctuate. If plating is conducted at high current densities, there are problems with burning or dendrites occurring on the electrodeposition coating. In addition, there are other problems, such as the substitution deposition of silver with respect to the plating substrate of copper or copper alloy and the like. In other words, deposition due to chemical substitution action based on oxidation-reduction electric potentials. Additionally, further substitution deposition of silver on top of the deposited silver alloy coating may occur. As a result, the silver or silver alloy plating coating does not achieve a fine and high-quality outer appearance.